1. Field of the Invention
The present invention relates generally to RF and microwave components, and particularly to balun devices.
2. Technical Background
A balun is a device that is employed in various types of applications to convert differential (balanced) signals to unbalanced signals, and vice-versa. A Balun may also function as a transformer and is often used as a means to change or match impedances within a portion of an RF network. Balanced signals require two transmission paths. One path carries a first signal and the second path carries a second signal that is of equal amplitude and opposite in phase to the first signal. This arrangement is typically employed to cancel the deleterious effects of noise and interference that might otherwise degrade a single-ended signal. On the other hand, there are certain components in RF and wireless devices, such as power amplifiers and antennas, that are typically implemented as single ended signal devices. Accordingly, a balun is required when a device includes single-ended and differential components in a signal path.
A very popular balun for use at RF and microwave frequencies is the Marchand balun. The Marchand balun typically includes two λ/4 coupled sections that are configured to provide balanced performance. In a symmetric TEM structure, the odd-mode and the even-mode impedances of the coupled structure define the transformation ratio from the single ended impedance to the differential impedance of the balun.
In one approach that has been considered, each of the λ/4 transmission line structures is disposed in a spiraled geometry to minimize the size of the balun in the X-Y plane and make the Marchand balun more compact. The size in the X-Y plane is further reduced, albeit at the expense of the z-dimension (i.e., profile height), by placing the coupled transmission line sections one atop the other. The typical market for such components will accommodate profile heights up to 1 mm for direct assembly onto circuit board and 0.4-0.6 mm for integration into RF modules. This approach, however, has drawbacks and limitations. In order to realize further size reductions, the dielectric constant of the dielectric material employed in the sandwiched structure must be increased to lower the quarter wavelength frequency. Unfortunately, this makes manufacturing tolerances more pronounced. The resultant balun usually exhibits a degraded performance because it is difficult to maintain an adequately high even-mode impedance and low DC resistance when the dielectric constant is high. Alternatively, if a lower dielectric constant is used, the conductor trace widths are smaller and the metal trace lengths longer. This approach, however, is unattractive because it increases insertion loss and DC resistance
What is needed therefore, is a compact balun that overcomes the deficiencies described above. In particular, what is needed is a compact balun that operates at a reduced commensurate frequency that is a fraction of the normal operating frequency of a Marchand balun of substantially the same or similar size. It is also desirable that the compact Marchand balun can be implemented with an arbitrarily selected transformation ratio.